High-density 3D graphene-based monolith and related materials, methods, and devices

ABSTRACT

A composition comprising at least one high-density graphene-based monolith, said monolith comprising a three-dimensional structure of graphene sheets crosslinked by covalent carbon bonds and having a density of at least 0.1 g/cm 3 . Also provided is a method comprising: preparing a reaction mixture comprising a suspension and at least one catalyst, said suspension selected from a graphene oxide (GO) suspension and a carbon nanotube suspension; curing the reaction mixture to produce a wet gel; drying the wet gel to produce a dry gel, said drying step is substantially free of supercritical drying and freeze drying; and pyrolyzing the dry gel to produce a high-density graphene-based monolith. Exceptional combinations of properties are achieved including high conductive and mechanical properties.

RELATED APPLICATIONS

This application claims priority to U.S. provisional application Ser.No. 61/745,528 filed Dec. 21, 2012, which is hereby incorporated byreference for all purposes.

FEDERAL FUNDING STATEMENT

The United States Government has rights in the invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND

Individual graphene sheets have attracted significant attention sincetheir isolation (Novoselov et al., Science 306, 666 (2004)), due to themultitude of graphene's exceptional properties (Gomez-Navarro et al.,Nano Letters 7, 3499 (2007); Geim et al., Nature Materials 6, 183(2007); Lee et al., Science 321, 385 (2008); Miller et al., Science 329,1637 (2010); Novoselov et al., Science 315, 1379 (2007); Schedin et al.,Nature Materials 6, 652 (2007); Zhu et al., Adv Mater 22, 3906 (2010);Chen et al., Nat Nano 3, 206 (2008).

Some of the properties observed in this nanoscale, two-dimensional (2D)form of graphitic carbon include room-temperature electricalconductivities up to ˜10⁶ S/cm and Young's moduli up to ˜10⁶ MPa, whichare among the highest reported for any material. In an effort to realizethe properties of individual graphene sheets on the macro-scale, severalgroups have recently developed three dimensional (3D) grapheneassemblies with many promising characteristics.

Such assemblies are comprised of randomly interconnected graphene sheetswith a large degree of porosity (>90%), which is believed to benecessary to minimize restacking of graphene sheets. These low-densitynanoporous graphene structures exhibit electrical conductivities andYoung's moduli as many as 10 orders of magnitude lower than thoseobserved for individual graphene sheets, which is not surprising giventheir high porosity. It is a direct consequence of superlineardependences of electrical and mechanical properties on the monolithdensity for porous materials.

Macro-scale 3D graphene-based materials exhibiting the exceptionalproperties, including combinations of properties, of graphene sheets arestill a challenge. For example, electrical conductivity might be toolow.

SUMMARY

Embodiments provided herein include compositions, devices, and articles,as well as methods of making and methods of using the compositions,devices, and articles.

One embodiment provides for a composition comprising at least onehigh-density graphene-based monolith, said monolith comprising athree-dimensional structure of graphene sheets crosslinked by covalentcarbon bonds and having a density of at least 0.2 g/cm³.

Another embodiment provides a method comprising: preparing a reactionmixture comprising a suspension and at least one catalyst, saidsuspension selected from a graphene oxide (GO) suspension and a carbonnanotube suspension; curing the reaction mixture to produce a wet gel;drying the wet gel to produce a dry gel, said drying step issubstantially free of supercritical drying and freezing drying; andpyrolyzing the dry gel to produce a high-density graphene-based monolithor a high-density carbon-nanotube-based monolith.

Another embodiment provides for a device comprising at least onehigh-density graphene-based monolith, said monolith comprising athree-dimensional structure of graphene sheets crosslinked by covalentcarbon bonds and having a density of at least 0.2 g/cm³.

Another embodiment provides for a composition comprising at least onehigh-density carbon-nanotube-based monolith, said monolith comprisingcarbon nanotubes crosslinked by nanoparticles and having a density of atleast 0.2 g/cm³.

Another embodiment provides a method comprising: preparing a reactionmixture comprising a suspension and at least one catalyst, saidsuspension selected from a graphene oxide (GO) suspension and/or acarbon nanotube suspension; curing the reaction mixture to produce a wetgel; drying the wet gel to produce a dry gel without application of heatto raise the temperature beyond ambient temperature; and pyrolyzing thedry gel to produce a high-density graphene-based monolith or ahigh-density carbon-nanotube-based monolith.

At least one advantage for at least one embodiment is that bulkproperties, such as Young's moduli, compressive strength, and electricalconductivity, can exceed those of commercial graphite at nearly half thedensity. Isotropic properties can approach those of graphene sheets.Another advantage for at least one embodiment is low annealingtemperature.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 shows a photograph of the graphene wet gel before (left) andafter (right) drying. For reference, the wet gel is in a 20 ml vial.

FIG. 2 shows FE-SEM images of a fracture surface of the GMA (a,b) andGDC (c,d) at low and high magnification.

FIG. 3 shows XRD spectra for GDC, GMA, GO powder and commercialgraphite.

FIG. 4 shows nanoindentation stress vs strain plot for GDC. Graphiteincluded for reference.

FIG. 5 shows uniaxial compression plot for GDC and graphite.

FIG. 6 shows log-log plot of Young's modulus as a function of density.

FIG. 7 shows log-log plot of conductivity as a function of density.

DETAILED DESCRIPTION

Introduction

References cited herein can be used to practice and better understandthe claimed inventions and are incorporated by reference herein in theirentireties for all purposes.

Priority U.S. provisional application Ser. No. 61/745,528 filed Dec. 21,2012 is hereby incorporated by reference in its entirety for allpurposes including working examples and claims.

US Patent Publication 2012/0034442 to Worsley et al., “MechanicallyStiff, Electrically Conductive Composites of Polymers and CarbonNanotubes” is incorporated herein by reference in its entirety.

US Patent Publication 2010/0187484 to Worsley et al., “MECHANICALLYROBUST, ELECTRICALLY CONDUCTIVE ULTRALOW-DENSITY CARBON NANOTUBE-BASEDAEROGELS” is incorporated herein by reference in its entirety.

The article, “Mechanically Robust 3D Graphene Macroassembly with HighSurface Area,” Worsley et al., Chem. Commun., 2012, 48, 8428-8430, isincorporated herein by reference in its entirety.

The article, “Synthesis of Graphene Aerogel with High ElectricalConductivity,” Worsley et al., J. Am. Chem. Soc., 2011, 2, 921-925, isincorporated herein by reference in its entirety.

The article, “High Surface Area, sp2-Cross-Linked Three-DimensionalGraphene Monolith,” Worsley et al., J. Phys. Chem. Letter, 2010,132(40), 14067-14069, is incorporated herein by reference in itsentirety.

The article, “Advanced Carbon Aerogels for Energy Applications,” Bieneret al., Energy & Environmental Science, 2011, 4, 656-667, isincorporated herein by reference in its entirety.

US Patent Publication 2010/0144904 describes graphene aerogel but thematerial is not crosslinked and is not air-dried.

High-Density Graphene-Based Monolith

A high-density graphene-based monolith described herein can be a blackmonolith that is mechanically robust, electrically conductive, and ofhigh density. Monolith is a term known in the art. Monolith and methodsfor making monolith are disclosed in, for example, U.S. Pat. No.5,207,814, U.S. Pat. No. 5,885,953, U.S. Pat. No. 5,879,744, U.S. Pat.No. 7,378,188, U.S. Pat. No. 7,410,718, and U.S. Pat. No. 7,811,711.

The high-density graphene-based monolith can comprise athree-dimensional structure of graphene sheets crosslinked by chemicallinkage such as covalent carbon-carbon bond. In one embodiment, 50% ormore, or 70% or more, or 90% or more of the crosslinking covalent bondsare sp² bonds.

In one embodiment, the high-density graphene-based monolith issubstantially free of graphene sheets interconnected only by physicalcrosslinks (e.g. Van der Waals forces). In one embodiment, less than10%, or less than 5% or less than 3%, or less than 1% of the graphenesheets are interconnected only by physical crosslinks.

In one embodiment, the high-density graphene-based monolith issubstantially free of graphene sheets interconnected only by metalcrosslinks (e.g., noble metal such as Pd). In one embodiment, less than10%, or less than 5% or less than 3%, or less than 1% of the graphenesheets are interconnected only by metal crosslinks.

In one embodiment, the high-density graphene-based monolith issubstantially free of graphene sheets with hydroxide or epoxidefunctionalities. In another embodiment, 10% or less, or 5% or less, or3% or less, or 1% or less, or 0.5% or less, or 0.1% or less of thecarbon atoms in the graphene-based monolith are connected to a hydroxideor epoxide functionality.

The high-density graphene-based monolith comprises, for example,restacking or substantial restacking of graphene sheets. The graphenesheets can have lateral dimensions of, for example, 10 nm or more, 50 nmor more, or 100 nm or more, including 10 nm to 50 nm. The surfaces ofthe graphene sheets can be, for example, free or substantially free ofnanoparticles.

In one embodiment, the high-density graphene-based monolith is not madeby stacking non-organic material, such as metals, between graphenesheets.

The high-density graphene-based monolith can have an atomic oxygencontent of, for example, 10% or less, or 5% or less, or 3% or less, or1% or less. The high-density graphene-based monolith can be mesoporous.The average pore size can be, for example, less than 20 nm.

The high-density graphene-based monolith can have a density of, forexample, 0.2 g/cm³ or more, or 0.5 g/cm³ or more, or 1.0 g/cm³ or more,or 1.5 g/cm³ or more.

The high-density graphene-based monolith can have a bulk electricalconductivity of, for example, 100 S/cm or more, or 200 S/cm or more, or500 S/cm or more, or 1,000 S/cm or more, or 1,500 S/cm or more, or 2,000S/cm or more.

The high-density graphene-based monolith can have a surface area of, forexample, 200 m²/g or more, 100 m²/g or more, or 70 m²/g or more, or 50m²/g or more, or m²/g or more.

The high-density graphene-based monolith can have a Young's modulus of,for example, 1 GPa or more, or 5 GPa or more, or 10 GPa or more.

The high-density graphene-based monolith can have a failure stress of,for example, 0.2 GPa or more, or 0.5 GPa or more, or 1 GPa or more.

The high-density graphene-based monolith can have a compressive strengthof, for example, 50 MPa or more, or 100 MPa or more, or 200 MPa or more.

High-Density Carbon Nanotube-Based Monolith

An alternative embodiment is for a high-density carbon nanotube-basedmonolith. In particular, a composition is provided comprising at leastone high-density carbon nanotube-based monolith, said monolithcomprising carbon nanotubes crosslinked by nanoparticles and having adensity of at least 0.2 g/cm³.

In some embodiments, the carbon nanotubes are single-walled carbonnanotubes or double-walled carbon nanotubes. More generally, the carbonnanotubes can be multi-walled nanotubes.

In one embodiment, the carbon-nanotube-based monolith comprises at least20 wt. % of carbon nanotubes. In one embodiment, thecarbon-nanotube-based monolith comprises at least 40 wt. % of carbonnanotubes. In one embodiment, the carbon-nanotube-based monolithcomprises at least 60 wt. % of carbon nanotubes. In one embodiment, thecarbon-nanotube-based monolith comprises at least 80 wt. % of carbonnanotubes.

In one embodiment, the monolith has a density of at least 0.2 g/cm³. Inone embodiment, the monolith has a density of at least 0.5 g/cm³. In oneembodiment, the monolith has a density of at least 0.7 g/cm³.

In one embodiment, the monolith has an electrical conductivity of atleast 200 S/cm or more. In one embodiment, the monolith has anelectrical conductivity of at least 400 S/cm or more.

In one embodiment, the monolith has a Young's modulus of at least 0.5GPa. In one embodiment, the monolith has a Young's modulus of at least 1GPa.

Process for Making High-Density Monolith

Precursors to graphene are known in the art. For example, graphene oxideis a general term for oxidized graphene, which can be a precursor tographene. Closely related precursors can include, for example, graphiteoxide, single layer graphene oxide, exfoliated graphite, and the like.See US 2012/0034442, incorporated by reference in its entirety.

Carbon nanotubes are known in the art. See US 2010/0187484, incorporatedby reference in its entirety. For example, the carbon nanotube can be asingle-walled carbon nanotube, a double-walled carbon nanotube, or amulti-walled carbon nanotube.

In some embodiments, the reaction mixture comprises a graphene oxide(GO) suspension. Methods for making GO are known in the art anddisclosed in, for example, Hummer, J. Am. Chem. Soc., 80:1339 (1958),which is incorporated herein by reference in its entirety. In oneembodiment, the GO suspension is an aqueous suspension. In anotherembodiment, the GO suspension is a suspension of at least one organicsolvent, such as alcohol, dimethylformamide, tetrahydrofuran, ethyleneglycol, N-methylpyrrolidone, etc. In one embodiment, the GO suspensionis an aqueous suspension made by sonicating GO in deionized water. Thetime for sonication can range from 0.25-24 hours. The concentration ofGO in the reaction mixture can be 0.1 mg/cm³ or more, or 1 mg/cm³ ormore, or 2 mg/cm³ or more, or 5 mg/cm³ or more, or 10 mg/cm³ or more.

In some embodiments, the reaction mixture comprises a carbon nanotubesuspension. See Worsley, et al., J. Mater. Chem., 19:3370 (2009), whichis incorporated herein by reference in its entirety. In one embodiment,the carbon nanotube suspension is an aqueous suspension. In anotherembodiment, the carbon nanotube suspension is a suspension of at leastone organic solvent. In one embodiment, the carbon nanotube suspensionis an aqueous suspension made by sonicating carbon nanotubes indeionized water. The time for sonication can range from 0.25-24 hours.The concentration of carbon nanotubes in the reaction mixture can be,for example, 0.1 to 80 wt. %, 0.1 to 60 wt. %, 0.1 to 50 wt. %, or0.1-32 wt. %. The upper percentage for the carbon nanotube concentrationis not particularly limited except by practicalities for a particularsystem.

The reaction mixture may also comprise additional reactant known for solgel reactions, though it is may not be necessary for gelation. In oneembodiment, the reaction mixture comprises resorcinol-formaldehyde (RF),phloroglucinol-formaldehyde, phenol-formaldehyde, cresol-formaldehyde,or phenol-furfuryl alcohol. In one embodiment, the reaction mixture isfree of RF. In another embodiment, the reaction mixture comprises RF. Ina particular embodiment, the GO-to-RF ratio is 0.1 or more.

The reaction mixture also can comprise at least one sol gel catalyst. Inone embodiment, the catalyst is an acid catalyst. In another embodiment,the catalyst is a base catalyst. An example is a hydroxide catalyst.Suitable catalysts include, but are not limited to, nitric acid, aceticacid, ascorbic acid, hydrochloric acid, sulfuric acid, sodium carbonate,sodium hydroxide, ammonium hydroxide, and calcium sulfate. Thereactant-to-catalyst ratio may range from 10 to greater than 1000.

High-density graphene-based monoliths and high-densitycarbon-nanotube-based monoliths can be made from the reaction mixtureaccording to the following steps: (i) curing the reaction mixture toproduce a wet gel; (ii) drying the wet gel to produce a dry gel, saidstep is substantially free of supercritical drying and freeze drying;and (iii) pyrolyzing the dry gel to produce a high-densitygraphene-based monolith or a high-density carbon-nanotube-basedmonolith.

In one embodiment, the reaction mixture is cured at a temperature of25-100° C. to produce a wet gel. In another embodiment, the reactionmixture is cured for 12-72 hours at a temperature of 85° C. In a furtherembodiment, the reaction mixture is cured at atmospheric pressure.

In one embodiment, the wet gel is subjected to solvent exchange toremove reaction by-products. Suitable solvent include, but are notlimited to, DI water. In another embodiment, the wet gel is subjected tosolvent exchange to remove water. Suitable solvents include, but are notlimited to, acetone.

In one embodiment, the wet gel is dried under ambient temperature andpressure. In one embodiment, the wet gel is dried under ambienttemperature and pressure for at least 24 hours. The application of heatcan be avoided. In particular, the drying step can comprise drying thewet gel to produce a dry gel without application of heat to raise thetemperature beyond ambient temperature. The drying step is carried outwithout supercritical drying. If desired, some heat can be applied forincreasing the speed of drying, but it is not essential or required inmany or all embodiments.

In one embodiment, the dry gel is pyrolyzed in an inert gas to produce ahigh-density graphene-based monolith or a high-densitycarbon-nanotube-based monolith. Suitable inert gases include, but arenot limited to, nitrogen. The pyrolyzing temperature can be 500° C. ormore, or 800° C. or more, or 1000° C. or more. The pyrolyzingtemperature can be, for example, less than 2,500° C., or less than2,000° C., or less than 1,500° C.

Applications

The monoliths, including high-density graphene-based monolith and thehigh-density carbon-nanotube-based monolith, described herein can beused in a variety of devices. For example, they can be used inelectrodes, batteries, capacitors, supercapacitors, sensors, actuators,membranes, structural elements, furnace components, catalyst supports,and hydrogen storage devices. In a particular embodiment, thehigh-density graphene-based monolith described herein is used in alithium battery electrode. Other applications can include, for example,lightweight armor, missile nose cone, and high temperature/thermal shockapplications.

In addition, graphene monoliths can also be used to shield sensitiveelectronic devices like cell phones, personal digital assistants,computers, and hospital equipment from Electromagnetic Interference(EMI) and Radio Frequency Interference (RFI). Shielding must beelectrically conductive and traditionally made of metal, which poses aweight problem for smaller electronics. Additionally, graphene monolithsare lightweight materials that outperform metal shielding. Positiveattributes of graphene monoliths in this application include lightweight, resistance to corrosion, and flexibility.

Additional embodiments are provided in the following non-limitingworking examples.

WORKING EXAMPLES Example 1 Material Synthesis

In a typical reaction, graphene oxide (GO) was suspended in deionizedwater and thoroughly dispersed using a VWR Scientific Model 75TAquasonic (sonic power ˜90 W, frequency ˜40 kHz). The concentration ofGO in the reaction mixture was 2 wt %. To determine the optimalconditions for GO dispersion, a range of sonication times (4 to 24 hrs)was evaluated. Once the GO (3 ml) was dispersed, concentrated ammoniumhydroxide (633 μl) was added to the dispersion. The sol-gel mixture wasthen transferred to glass molds, sealed and cured in an oven at 85° C.for 12-72 h. The resulting gels were then removed from the molds andwashed with deionized water to remove reaction byproducts and excessammonium hydroxide. Then the gels were washed in acetone to remove allthe water from the pores of the gel network. The wet gels weresubsequently dried under ambient conditions to yield the high-densityreduced GO assemblies. Some samples were also dried using supercriticalCO₂ to produce low-density reduced GO assemblies for comparison. Thereduced GO assemblies were thermally reduced via pyrolysis at 1050° C.under a N₂ atmosphere for 3 h. The 3D graphene macroassemblies wereisolated as black cylindrical monoliths.

Characterization

Field-emission scanning electron microscopy (SEM) characterization wasperformed on a JEOL 7401-F at 5-10 keV (20 mA) in secondary electronimaging mode with a working distance of 2-8 mm. Surface areadetermination were performed by Brunauer-Emmett Teller (BET) andBarrett-Joyner-Halenda (BJH) methods using an ASAP 2000 Surface AreaAnalyzer (Micromeritics Instrument Corporation) (Gregg et al.Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982).Samples of approximately 0.1 g were heated to 150° C. under vacuum (10⁻⁵Torr) for at least 24 hours to remove all adsorbed species. X-raydiffraction (XRD) measurements were performed on a Bruker AXS D8 ADVANCEX-ray diffractometer equipped with a LynxEye 1-dimensional linear Sistrip detector. The samples were scanned from 5 to 75° 2θ. The step scanparameters were 0.02° steps and 2 s counting time per step with a 0.499°divergence slit and a 0.499° antiscatter slit. The X-ray source wasNi-filtered Cu radiation from a sealed tube operated at 40 kV and 40 mA.Phases in the samples were identified by comparison of observed peaks tothose in the International Centre for Diffraction Data (ICDD PDF2009)powder diffraction database, and also peaks listed in referencearticles. Goniometer alignment was ensured using a Bruker-supplied Al₂O₃standard.

Monoliths were machined with a 6-mm-diameter cylindrical endmillrotating at a speed of 2×10⁴ revolutions per minute, yieldingmacroscopically flat surfaces needed for mechanical characterization byindentation. The samples were indented in the load-controlled mode in anMTS XP nanoindenter with a flat punch diamond tip with an effectivediameter of 62 microns. Representative indentation stress (σ) and strain(ε) were defined as σ=4P/(πD²) (i.e., the average contact pressure) andε=4h/(πD)≈h/D (i.e., the proportionality coefficient between σ and thereduced modulus in the elastic regime) (Kucheyev et al., Acta Materialia57, 3472, 2009). Here, P is the load, D is the indenter tip diameter,and h is the indenter displacement. Both loading and unloading rateswere kept constant to maintain an indentation strain rate of 10⁻³ s⁻¹(Kucheyev et al., Acta Materialia 57, 3472, 2009). Elastic propertiesare characterized by the Young's modulus, which was calculated based onthe initial slope of the unloading curve according to the Oliver-Pharrmethod (Oliver et al., Journal of Materials Research 7, 1564, 1992) formaximum loads below those resulting in failure events. In Oliver-Pharrcalculations, we assumed Poisson's ratios of diamond and aerogels of0.07 and 0.2, respectively, and the Young's modulus of diamond of 1141GPa (Kucheyev et al., Acta Materialia 57, 3472, 2009). Several (>10)measurements of the Young's modulus, failure stress, and failure strainwere made on different sample locations, and results were averaged. Theerror bars given are standard deviations. Electrical conductivity wasmeasured using the four-probe method with metal electrodes attached tothe ends of cylindrical samples. The amount of current transmittedthrough the sample during measurement was 100 mA, and the voltage dropalong the sample was measured over distances of 3 to 6 mm. Seven or moremeasurements were taken on each sample, and results were averaged. Bulkdensities of the samples were determined from the physical dimensionsand mass of each sample.

Analysis

Described here is a straightforward relatively low-temperature method torealize macroscopic 3D graphene-derived carbons (GDCs) with isotropicproperties approaching those of graphene sheets. Important factors forassembling a macrostructure with such properties are (i) the developmentof strong covalent links between graphene sheets that facilitate bothelectrical conductivity and structural reinforcement, (ii) sufficientrestacking of graphene sheets to reach relatively high densities of ˜1g/cm³, and (iii) an assembly that can be carbonized at relatively lowtemperatures (˜1000° C.).

In order to form strong links between graphene sheets in GDCs, a methodbased on chemical cross-linking of individual sheets of graphene oxide(GO) suspended in water was used. Such cross-linking involves various GOfunctional groups (e.g., epoxide and hydroxide), yielding a reduced GOgel. Similar approach was used in the synthesis of exceptionally stiffand electrically conductive low-density graphene macroassemblies (GMAs)(Worsley et al., Chem Commun, 48, 8428 (2012)). The restacking ofgraphene sheets in the GDC was achieved by simply drying the reduced GOgel under ambient conditions instead of using a supercritical solventextraction method that preserves the low-density structure and minimizessheet restacking in GMAs. The dried monoliths were then annealed atabout 1000° C. to reduce the organic cross-links to sp² carboncross-links. Resultant GDCs displayed isotropic bulk properties such aselectrical conductivities and Young's moduli that are 3-6 orders ofmagnitude higher than for any previously reported 3D graphene assembly,and exceeding isotropic graphite that is nearly twice as dense.

Synthesis of GDCs was carried out via gelation of a GO suspension underbasic conditions. Full experimental details is provided above in theMaterial Synthesis section. Briefly, in a typical synthesis,ultrasonication is used to disperse 2 wt % GO in deionized water.Concentrated NH₄OH was added (211 μl/g GO suspension) to the suspension,which was then sealed and placed in an oven at 85° C. to gel. Aftergelation, the reduced GO gel was washed first in deionized water andthen in acetone. After that, the gel was allowed to dry under ambientconditions for at least 24 hours, followed by annealing at 1050° C.under nitrogen to yield the final GDC with a monolith density of ˜1.0g/cm³. This is an ˜10-fold increase in the density compared to GMAs thathave densities of ˜0.10 g/cm³. Importantly, the randomly orientatedself-assembly during gelation combined with the capillary force-inducedshrinkage led to the formation of a completely isotropic material. FIG.1 shows monoliths immediately after gelation, after supercriticaldrying, and after ambient drying to illustrate the shrinkage thatoccurred upon solvent removal. There are few limits on the shapes andsizes of the GDCs produced since, in addition to being readilymechanically machined, the GDC conforms to the mold in which the initialreaction mixture is placed.

The difference between the microstructure of GDCs and GMAs wasillustrated by scanning electron microscopy (SEM) images in FIG. 2. GMAswere made of randomly interconnected graphene sheets with a minimalrestacking (FIG. 2a-b ) resulting in fine sheet-like features and alarge porosity of ˜95% (Bai et al., The Journal of Physical Chemistry C115, 5545 (2011); Chen et al., Nanoscale 3, 3132 (2011); Sui et al.,Carbon 49, 4314 (2011); Wang et al., ECS Transactions 19, 241 (2009);Worsley et al., Chem Commun, 48, 8428 (2012); Worsley et al., J. Am.Chem. Soc. 132, 14067 (2010); Xu et al., ACS Nano 4, 4324 (2010); Yin etal., Small, 8, 16, 2458 (2012)).

In contrast, no fine sheet-like features were distinguishable in the GDCin FIG. 2c-d . However, the SEM shows that the GDC was homogenous andcontained many small pores (less than ˜20 nm). These pores contributedto the material's density being roughly half of the density for singlecrystalline graphite made of perfectly stacked graphene sheets (2.2g/cm³). This apparently open-cell porosity of the GDC is consistent withthe measured Brunauer-Emmett-Teller (BET) surface area of 69 m²/g.

The x-ray diffraction (XRD) patterns for GDCs and GMAs also showedsignificant deviations (FIG. 3). The appearance of peaks at ˜12° and26.5° for the GDC contrasted sharply with their absence in thelow-density analog. For the GMA, XRD revealed that only single andfew-layer sheets exist (Worsley et al., Chem Commun, 48, 8428 (2012)),evidenced by the absence of the (002) diffraction peak (at 26.5°)assigned to the basal spacing of graphite (0.35 nm). However, the GDCshowed a strong (002) peak at 26.5°, indicating significant graphenesheet restacking, which is consistent with an increased monolithdensity, though not to the degree observed in commercial graphite. Inaddition to the (002) peak, the XRD pattern of the GDC exhibited a peakat ˜12° suggesting that there was a fraction of sheets with asubstantially larger interlayer spacing of ˜0.74 nm. As the O content ofthe GDC was less than 2%, (FIG. 4), same as for the GMA, it is unlikelythat this peak at ˜12° resulted from incomplete reduction as might besupposed given its proximity to the 001 reflection of graphene oxide.Instead, this larger spacing might be attributed to incomplete andimperfect restacking caused by random orientation and curvature ofgraphene sheets, resulting from our sheet-to-sheet cross-linking scheme.It is possible that this larger spacing further contributed to the lower(relative to graphite) GDC density of ˜1 g/cm³.

Despite having a density of close to half of that of perfectly stackedgraphene sheets (2.2 g/cm³) or even commercial grade graphite (1.6-1.9g/cm³), the mechanical properties of the GDC exceeded those of graphite.Nanoindentation results (FIG. 5) showed that the GDC exhibits a Young'smodulus of ˜10 GPa and a failure stress of ˜1.2 GPa independent of theloading direction. These exceeded respective nanoindentation values of8.5 GPa and 0.4 GPa (also independent of loading direction) forcommercial grade graphite (seehttp://www.mcmaster.com/#graphite-rods/=kkbdgu) with a much largerdensity of 1.7 g/cm³. Uniaxial compression results also showed that theGDC has a high compressive strength of ˜200 MPa compared to only ˜98 MPafor the commercial graphite material. The measured compressive strengthfor commercial graphite is consistent with reported values of 40-193MPa, depending on the material density (1.6-1.9 g/cm³) and defectcontent (see http://www.mcmaster.com/#graphite-rods/=kkbdgu;http://www.poco.com/tabid/70/Default.aspx;http://www.roccarbon.com/cgrawmaterials.html).

Considering that Young's modulus depends superlinearly on monolithdensity, ρ, for nanoporous materials with an exponent, n, in the rangeof 2-4, FIG. 6 shows a density dependence of the modulus for a number ofcarbon materials, including carbon aerogels (Pekala et al., Journal ofNon-Crystalline Solids 125, 67 (1990)), carbon-nanotube (CNT) assemblies(Worsley et al., Applied Physics Letters 94, 7, 073115 (2009)), graphene(Lee et al, Science 321, 385 (2008); Koenig et al., Nat Nano 6, 543(2011)), graphene-based materials (Worsley et al., Chem Commun, 48, 8428(2012); Tang et al., Angew Chem Int Edit 49, 4603 (2010); Zhang et al.,J Mater Chem 21, 6494 (2011)), and graphite. FIG. 6 shows that themodulus of the GDC scales with density as expected for CNT- orgraphene-based materials with n=2.5.

FIG. 6 suggests that if the density of the GDC could be furtherincreased to ˜2 g/cm³, the modulus would come within an order ofmagnitude of that for an individual atomically perfect graphene sheet(10⁶ MPa). The scaling of the modulus to a value lower than that of anatomically perfect graphene sheet probably stems from random orientationand curvature of graphene sheets as well as lattice defects.Nevertheless, it is quite remarkable that the GDC exhibited propertiesso close to those expected based on the density scaling of an assemblyof atomically perfect graphene sheets.

The bulk electrical conductivity of the GDC, determined via thefour-probe method, was 1,750 S/cm, which is likely larger than for any3D graphene assembly reported. FIG. 7 plots the density dependence ofthe electrical conductivity of the GDC along with other reported 3Dgraphene (Worsley et al., Chem Commun, 48, 8428 (2012); Worsley et al.,J. Am. Chem. Soc. 132, 14067 (2010); Zhang et al., J Mater Chem 21, 6494(2011)) and CNT assemblies (Worsley et al., Applied Physics Letters 94,7, 073115 (2009)), carbon aerogels (Worsley et al., Applied PhysicsLetters 94, 7, 073115 (2009)), graphite, and individual graphene sheets(Chen et al., Nat Nano 3, 206 (2008); Wu et al., ACS Nano 3, 411(2009)). Similar to the scaling of Young's modulus discussed above,electrical conductivity, σ, scales superlinearly with the materialdensity (σ˜ρ^(1.5)). FIG. 7 shows that, despite pronounced restacking ofgraphene sheets revealed by an increased density and combined SEM, XRD,and BET observations, the GDC follows the scaling law for CNT- andgraphene-based materials, extrapolating to a conductivity of ˜10⁴ S/cmat ˜2 g/cm³. This result is approximately 2 orders of magnitude lessthan what was observed for a perfect graphene sheet, highlighting howsensitive the electrical conductivity is to defects, sheet curvature,sheet stacking, and orientation (in-plane vs. through-plane). A scaledGDC conductivity of ˜10⁴ S/cm is, however, consistent with experimentalconductivity values for individual graphene sheets thermally reducedfrom GO (Wu et al., ACS Nano 3, 411 (2009)) and is an order of magnitudehigher than that of commercial graphite.

Example 2

In a typical reaction, purified CNTs were suspended in deionized waterand thoroughly dispersed using a VWR Scientific Model 75T Aquasonicsonication bath (sonic power ˜90 W, frequency ˜40 kHz). Theconcentration of CNTs in the reaction mixture ranged from 0 wt % to 2 wt%. To determine the optimal conditions for CNT dispersion, a range ofsonication times (4 to 24 hrs) were evaluated. Once the CNTs weredispersed, resorcinol (1.235 g, 11.2 mmol), formaldehyde (1.791 g, 22.1mmol), and sodium carbonate catalyst (5.95 mg, 0.056 mmol) were added tothe reaction solution. The resorcinol to catalyst ratios (R/C) employedfor the synthesis of the composites range from 50 to 200. The sol-gelmixture was then transferred to glass molds, sealed and cured in an ovenat 85° C. for 72 h. The resulting gels were then removed from the moldsand washed with acetone for 72 h to remove water from the pores of thegel network. The wet gels were subsequently dried under ambientconditions and pyrolyzed at 1050° C. under a N₂ atmosphere for 3 h. Thecomposite materials were isolated as black cylindrical monoliths. Foamswith CNT loadings ranging from 0 to ˜90 wt %, or ranging from 0 to ˜99wt %, were prepared by this method. The rest of the material can be, inmany cases, carbon aerogel particles that “glue” the CNTs together.These particles can, it is believed, come from theresorcinol-formaldehyde (RF) solution in which the CNTs are originallysuspended and gelled. The RF forms an organic gel which connects theCNTs. The organic “glue” is converted to carbon during pyrolysis at, forexample, 1050° C. in nitrogen.

Hence, described herein in some embodiments is a straightforward methodto realize high-density graphene-based macroassemblies with isotropicmechanical and electronic transport properties approaching those ofindividual graphene sheets. The design strategy involved, in someembodiments, direct cross-linking of graphene sheets via the functionalgroups in graphene oxide and sufficient restacking of the graphenesheets to increase the material density. The nature of the synthesis ledto a much lower temperature used than for the synthesis of commercialisotropic graphite and facilitated fabrication with few limits on thesize or shape. The development of 3D bulk macrostructures retainingproperties of individual graphene sheets will both expand and acceleratethe commercialization of graphene-based technologies and products.

Finally, the following references can be referred to practicingembodiments described herein and some are noted in FIGS. 6 and 7.

REFERENCES

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What is claimed is:
 1. A composition comprising at least onehigh-density graphene-based monolith, said monolith comprising athree-dimensional structure of restacked graphene sheets directlycrosslinked by covalent carbon bonds, wherein the monolith has anelectrical conductivity of at least 500 S/cm and a density of at least0.2 g/cm³, wherein less than 10% of the graphene sheets areinterconnected only by van der Waals force, and wherein the monolith hasa Young's modulus of at least 5 GPa.
 2. The composition of claim 1,wherein the surfaces of the graphene sheets are free of carbonnanoparticles.
 3. The composition of claim 1, wherein in the monolith atleast 70% of the covalent carbon bonds between the graphene sheets aresp² bonds.
 4. The composition of claim 1, wherein the monolith has adensity of at least 0.5 g/cm³.
 5. The composition of claim 1, whereinthe monolith has an electrical conductivity of at least 1000 S/cm. 6.The composition of claim 1, wherein the monolith has a failure stress ofat least 0.5 GPa.
 7. The composition of claim 1, wherein the monolithhas a compressive strength of at least 100 MPa.
 8. The composition ofclaim 1, wherein the monolith has (i) a density of at least 0.5 g/cm³,(ii) an electrical conductivity of at least 1000 S/cm, and (iii) aYoung's modulus of at least 5 GPa.
 9. A device comprising thecomposition of claim
 1. 10. The device according to claim 9, wherein themonolith wherein monolith is incorporated into an electrode.
 11. Thedevice according to claim 9, wherein the monolith has (i) a density ofat least 0.5 g/cm³, (ii) an electrical conductivity of at least 1000S/cm, and (iii) a Young's modulus of at least 5 GPa.
 12. The deviceaccording to claim 9, wherein the device is a capacitor, a battery, anelectrode, a sensor, an actuator, a membrane, a catalyst support, or ahydrogen storage device.
 13. A method for obtaining the composition ofclaim 1, comprising: preparing a reaction mixture comprising asuspension and at least one catalyst, said suspension is a grapheneoxide (GO) suspension; curing the reaction mixture to produce a wet gel;drying the wet gel to produce a dry gel, said drying step issubstantially free of supercritical drying and freezing drying; andpyrolyzing the dry gel to produce the composition comprising ahigh-density graphene-based monolith.
 14. The method of claim 13,wherein the suspension is a GO suspension comprising water or at leastone organic solvent.
 15. The method of claim 13, wherein the suspensionis a GO suspension comprising water or at least one organic solvent, andwherein the catalyst comprises ammonium hydroxide.
 16. The method ofclaim 13, wherein the reaction mixture is cured at a temperature of 100°C. or less.
 17. The method of claim 13, wherein the step of drying thewet gel comprises solvent exchange.
 18. The method of claim 13, whereinthe step of drying the wet gel comprises washing the wet gel withdeionized water and subsequently with acetone.
 19. The method of claim13, wherein the step of drying the wet gel comprises drying the wet gelunder ambient condition.
 20. The method of claim 13, wherein the step ofpyrolyzing the dry gel comprises annealing the dry gel in an inertatmosphere at a temperature of 500° C. or higher.
 21. The method ofclaim 13, further comprising incorporating the high-densitygraphene-based monolith into an electrode.